Diabetes mellitus is a metabolic disease that currently affects approximately 5% of the general population in Western countries. Type 2 diabetes mellitus accounts for 90-95% of all diabetes cases. In the last decades, the prevalence of type 2 diabetes has increased steeply due to a combination of ageing, unhealthy diets, ... read more obesity and increasingly inactive lifestyles, as well as an earlier identification of patients with diabetes. Adequate treatment of type 2 diabetes is important to prevent or delay micro- and macrovascular complications. This thesis deals with both the adherence of physicians to treatment guidelines and the adherence of patients to their medication regimens. Treatment guidelines are based on Randomized Clinical Trials, which are considered as the gold standard to establish the efficacy of treatments. However, they may not reflect actual drug use in daily clinical practice and may not reflect real life clinical effectiveness. This thesis contains five observational cohort studies to describe long term patterns of medication use and outcomes of treatment in daily clinical practice. Furthermore it contains a study on patients’ information needs concerning oral antidiabetic medication show less